Bipolar power transistors in the Darlington configuration have the drawback of being very slow to switch on and off. In fact, at the moment of switching, the speed-up current of the charges stored in the base region has an immediate effect only on the driver transistor (T1) and a slower effect on the final transistor (T2).
The most commonly used means for speeding up the switching of a Darlington is to insert a speed-up diode between the base of the driver and that of the final transistor, as shown in the diagram of FIG. 1. This diode can be made integral with a very low gain speed-up transistor (T3), as shown in the diagram of FIG. 2.
In a known Darlington device, the base of the speed-up transistor T3 is highly-doped P++ in order to considerably lower the relative gain.
The known device entails the following drawbacks:
the step of diffusing the base of T3 is separate from the step of obtaining the bases of T1 and T2;
there is an unevenness in the gain of T3, due to the fact that it is not easy to control the base doping since it is deposited from a solid source;
there is an introduction of heavy metals due to the use of a solid doping source, with a consequent reduction of the lifetime and current characteristics of the device;
there is a greater probability of damage to the surface of the silicon due to the deposition of very large doses of dopant (boron).